WO 2010/064911 describes how to generate a layer of micro bubbles on the hull of a displacement vessel in order to reduce frictional drag, by means of a cavity being provided in a flat bottom area of the hull of a vessel, such that an opening of the cavity is at the flat bottom area. Air is injected into the cavity at such a rate that the water level in the cavity is kept substantially at the level of the hull of the vessel. Due to the forward movement of the vessel, the air in the cavity will move relative to the water at the speed at which the vessel is moving, or seen from the cavity, the water will flow past the cavity at that speed. This difference in velocity between the air and the water causes a so-called Kelvin Helmholtz Instability (KHI) which results in a mixing of air and water at the interface between the water and the air, and in a consequential generation of a layer of small sized air bubbles. The small size of these air bubbles makes them very stable and they tend to stay in the water for a relative long period. The small bubbles generated in the cavity are released there from at the rear edge of the cavity such that they form a stable lubricating layer extending a long distance along the bottom of the hull downstream of the cavity. In this way energy savings in propulsion of a vessel can be achieved.
WO 2015/133900 describes providing a number of wave deflectors that extend transversely across the cavity and are connected to sidewalls of the cavity, wherein the wave deflectors are arranged for facilitating emptying of an air cavity during start up. The wave deflectors reduce turbulence inside the cavity and cause air to be retained inside the cavity for a longer period of time such that compressors of reduced capacity for less powerful air injection during start up are required. By providing a wave deflector inside the cavity with a relatively long dimension in the cavity length direction, the cavity can be filled with air effectively while the vessel is sailing, e.g. at a speed of 20 knots. In operation, the deflector effectively shields the air-filled cavity from water entry due to waves and roll motions of the vessel, the elongate deflector part keeping the water surface inside the cavity stable during roll motions, such that a stable operation of the cavity is warranted. The deflectors are welded to the sidewalls
Though an air lubrication system provided with such wave deflectors significantly reduces the drag on the hull of a vessel when in operation, i.e. when providing the air lubricating layer, when the system is not in operation the air cavities increase the drag on the hull.
It is an object of the present invention to provide an air lubrication system and vessel with reduced drag on the hull of the vessel, even when the air lubrication system is switched off.
It is a further object to provide an air lubrication system which is less susceptible to debris becoming lodged in the air cavity of the air lubrication system.
The present invention further aims to provide an air lubrication system in a relatively low volume of air suffices for expelling water from the air cavity